How can I use the Blender compositor to blur multiple objects in a scene?
Instead of using multiple identical Blur nodes and combining the result you should combine the outputs from the ID Mask nodes and use that as te input to a single Blur. You can easily combine them using Math nodes set to Maximum, chaining one to the other until you have combined all ID masks and finally into the single Blur. Since there is only one Blur this should also be considerably more efficient to render.



To allow the blurring of each object index to be controlled individually you can add some additional maths nodes to multiply each by a variable value (shown in Cyan) as follows :



This can produce the following result :



You can keyframe the Value inputs to vary the blur for each ID - with 0.0 for no blur, up to 1.0 for full blur.